[Illustration: _Fig. 1._
The Solar System.]
METEORIC ASTRONOMY:
A TREATISE
ON
SHOOTING-STARS, FIRE-BALLS,
AND
AEROLITES.
BY
DANIEL KIRKWOOD, LL.D.
PROFESSOR OF MATHEMATICS IN WASHINGTON AND JEFFERSON COLLEGE.
[Illustration]
PHILADELPHIA:
J. B. LIPPINCOTT & CO.
1867.
Entered, according to Act of Congress, in the year 1867, by
DANIEL KIRKWOOD, LL.D.,
In the Clerk's Office of the District Court of the United States
for the Western District of Pennsylvania.
PREFACE.
Aristotle and other ancient writers regarded comets as meteors
generated in the atmosphere. This opinion was generally accepted, even
by the learned, until the observations of Tycho, near the close of the
sixteenth century, showed those mysterious objects to be more distant
than the moon, thus raising them to the dignity of _celestial_ bodies.
An achievement somewhat similar, and certainly no less interesting,
was reserved for the astronomers of the _nineteenth_ century. This was
the great discovery that _shooting-stars, fire-balls, and meteoric
stones, are, like comets, cosmical bodies moving in conic sections
about the sun_. DR. HALLEY was the first to foretell the return of a
comet, and the year 1759 will ever be known in history as that which
witnessed the fulfillment of his prophecy. But in the department of
_meteoric_ astronomy, a similar honor must now be awarded to the late
DR. OLBERS. Soon after the great star-shower of 1833 he inferred from
a comparison of recorded facts that the November display attains
a maximum at intervals of thirty-three or thirty-four years. He
accordingly designated 1866 or 1867 as the time of its probable return;
and the night of November 13th of the former year must always be
memorable as affording the first verification of _his_ prediction. On
that night several thousand meteors were observed in one hour from a
single station. This remarkable display, together with the fact that
another still more brilliant is looked for in November, 1867, has
given meteoric astronomy a more than ordinary degree of interest in
the public mind. To gratify, in some measure, the curiosity which has
been awakened, by presenting in a popular form the principal results of
observation and study in this new field of research, is the main design
of the following work.
The first two chapters contain a popular view of what is known in
regard to the star-showers of August and November, and also of some
other epochs. The third is a description, in chronological order,
of the most important falls of meteoric stones, together with the
phenomena attending their descent. The fourth and following chapters
to the eleventh inclusive, discuss various questions in the theory
of meteors: such, for instance, as the relative number of aerolitic
falls during different parts of the day, and also of the year; the
coexistence of the different forms of meteoric matter in the same
rings; meteoric dust; the stability of the solar system; the doctrine
of a resisting medium; the extent of the atmosphere as indicated by
meteors; the meteoric theory of solar heat; and the phenomena of
variable and temporary stars. The twelfth chapter regards the rings
of Saturn as dense meteoric swarms, and accounts for the principal
interval between them. The thirteenth presents various facts, not
previously noticed, respecting the asteroid zone between Mars and
Jupiter, with suggestions concerning their cause or explanation.
As the nebular hypothesis furnishes a plausible account of the origin
of meteoric streams, it seemed desirable to present an intelligible
view of that celebrated theory. This accordingly forms the subject of
the closing chapter.
The greater part of the following treatise, it is proper to remark, was
written before the publication (in England) of Dr. Phipson's volume on
"Meteors, Aerolites, and Falling-stars." The author has had that work
before him, however, while completing his manuscript, and has availed
himself of some of the accounts there given of recent phenomena.
CANONSBURG, PA, _May, 1867_.
CONTENTS.
PAGE
INTRODUCTION 7
CHAPTER I.
The Meteors of November 12th-14th 13
CHAPTER II.
Other Meteoric Rings 26
CHAPTER III.
Aerolites 35
CHAPTER IV.
Conjectures in Regard to Meteoric Epochs 50
CHAPTER V.
Geographical Distribution of Meteoric Stones--Do
Aerolitic Falls occur more frequently by Day than by
Night?--Do Meteorites, Bolides, and the matter of ordinary
Shooting-stars, coexist in the same Rings? 56
CHAPTER VI.
Phenomena supposed to be Meteoric--Meteoric Dust--Dark Days 65
CHAPTER VII.
Researches of Reichenbach--Theory of Meteors--Stability of
the Solar System--Doctrine of a Resisting Medium 74
CHAPTER VIII.
Does the Number of Aerolitic Falls vary with the Earth's
Distance from the Sun?--Relative Numbers observed in the
Forenoon and Afternoon--Extent of the Atmosphere as indicated
by Meteors 79
CHAPTER IX.
The Meteoric Theory of Solar Heat 84
CHAPTER X.
Will the Meteoric Theory account for the Phenomena of
Variable and Temporary Stars? 92
CHAPTER XI.
The Lunar and Solar Theories of the Origin of Aerolites 96
CHAPTER XII.
The Rings of Saturn 102
CHAPTER XIII.
The Asteroid Ring between Mars and Jupiter 105
CHAPTER XIV.
Origin of Meteors--The Nebular Hypothesis 112
APPENDIX 123
INTRODUCTION.
A GENERAL VIEW OF THE SOLAR SYSTEM.
THE SOLAR SYSTEM consists of the sun, together with the planets and
comets which revolve around him as the center of their motions. The sun
is the great controlling orb of this system, and the source of light
and heat to its various members. Its magnitude is one million four
hundred thousand times greater than that of the earth, and it contains
more than seven hundred times as much matter as all the planets put
together.
MERCURY is the nearest planet to the sun; its mean distance being about
thirty-seven millions of miles. Its diameter is about three thousand
miles, and it completes its orbital revolution in 88 days.
VENUS, the next member of the system, is sometimes our morning and
sometimes our evening star. Its magnitude is almost exactly the same as
that of the earth. It revolves round the sun in 225 days.
THE EARTH is the third planet from the sun in the order of distance;
the radius of its orbit being about ninety-five millions of miles. It
is attended by one satellite--the moon--the diameter of which is 2160
miles.
MARS is the first planet exterior to the earth's orbit. It is
considerably smaller than the earth, and has no satellite. It revolves
round the sun in 687 days.
THE ASTEROIDS.--Since the commencement of the present century a
remarkable zone of telescopic planets has been discovered immediately
exterior to the orbit of Mars. These bodies are extremely small; some
of them probably containing less matter than the largest mountains on
the earth's surface. More than ninety members of the group are known at
present, and the number is annually increasing.
JUPITER, the first planet exterior to the asteroids, is nearly five
hundred millions of miles from the sun, and revolves round him in a
little less than twelve years. This planet is ninety thousand miles in
diameter and contains more than twice as much matter as all the other
planets, primary and secondary, put together. Jupiter is attended by
four moons or satellites.
SATURN is the seventh planet in the order of distance--counting the
asteroids as one. Its orbit is about four hundred millions of miles
beyond that of Jupiter. This planet is attended by eight satellites,
and is surrounded by three broad, flat rings. Saturn is seventy-six
thousand miles in diameter, and its mass or quantity of matter is more
than twice that of all the other planets except Jupiter.
URANUS is at double the distance of Saturn, or nineteen times that
of the earth. Its diameter is about thirty-five thousand miles, and
its period of revolution, eighty-four years. It is attended by four
satellites.
NEPTUNE is the most remote known member of the system; its distance
being nearly three thousand millions of miles. It is somewhat larger
than Uranus; has certainly one satellite, and probably several more.
Its period is about one hundred and sixty-five years. A cannon-ball
flying at the rate of five hundred miles per hour would not reach the
orbit of Neptune from the sun in less than six hundred and eighty years.
These planets all move round the sun in the same direction--from west
to east. Their motions are nearly circular, and also nearly in the same
plane. Their orbits, except that of Neptune, are represented in the
frontispiece. It is proper to remark, however, that all representations
of the solar system by maps and planetariums must give an exceedingly
erroneous view either of the magnitudes or distances of its various
members. If the earth, for instance, be denoted by a ball half an
inch in diameter, the diameter of the sun, according to the same
scale (sixteen thousand miles to the inch), will be between four and
five feet; that of the earth's orbit, about one thousand feet; while
that of Neptune's orbit will be nearly six miles. To give an accurate
representation of the solar system at a single view is therefore
plainly impracticable.
COMETS.--The number of comets belonging to our system is unknown. The
appearance of more than seven hundred has been recorded, and of this
number, the elements of about two hundred have been computed. They move
in very eccentric orbits--some, perhaps, in parabolas or hyperbolas.
THE ZODIACAL LIGHT is a term first applied by Dominic Cassini, in
1683, to a faint nebulous aurora, somewhat resembling the milky-way,
apparently of a conical or lenticular form, having its base toward
the sun, and its axis nearly in the direction of the ecliptic. The
most favorable time for observing it is when its axis is most nearly
perpendicular to the horizon. This, in our latitudes, occurs in March
for the evening, and in October for the morning. The angular distance
of its vertex from the sun is frequently seventy or eighty degrees,
while sometimes, though rarely (except within the tropics), it exceeds
even one hundred degrees.
The zodiacal light is probably identical with the meteor called
_trabes_ by _Pliny_ and _Seneca_. It was noticed in the latter part
of the sixteenth century by Tycho Brahé, who "considered it to be an
abnormal spring-evening twilight." It was described by Descartes
about the year 1630, and again by Childrey in 1661. The first accurate
description of the phenomenon was given, however, by Cassini. This
astronomer supposed the appearance to be produced by the blended
light of an innumerable multitude of extremely small planetary bodies
revolving in a ring about the sun. The appearance of the phenomenon as
seen in this country is represented in Fig. 2.
[Illustration: Fig. 2.]
For general readers it may not be improper to premise the following
explanations:
Meteors are of two kinds, _cosmical_ and _terrestrial_: the former
traverse the interplanetary spaces; the latter originate in the earth's
atmosphere.
_Bolides_ is a general name for meteoric fire-balls of greater
magnitude than shooting-stars.
The _period_ of a planet, comet, or meteor is the time which it
occupies in completing one orbital revolution.
The motion of a heavenly body is said to be _direct_ when it is from
west to east; and _retrograde_ when it is from east to west.
_Encke's Hypothesis of a Resisting Medium._--The time occupied by
Encke's comet in completing its revolution about the sun is becoming
less and less at each successive return. Professor Encke explains
this fact by supposing the interplanetary spaces to be filled with an
extremely rare fluid, the resistance of which to the cometary motion
produces the observed contraction of the orbit.
METEORIC ASTRONOMY.
CHAPTER I.
SHOOTING-STARS.
I. The Meteors of November 12th-14th.
Although shooting-stars have doubtless been observed in all ages of the
world, they have never, until recently, attracted the special attention
of scientific men. The first exact observations of the phenomena were
undertaken, about the close of the last century, by Messrs. Brandes and
Benzenberg. The importance, however, of this new department of research
was not generally recognized till after the brilliant meteoric display
of November 13th, 1833. This shower of fire can never be forgotten
by those who witnessed it.[1] The display was observed from the West
Indies to British America, and from 60° to 100° west longitude from
Greenwich. Captain Hammond, of the ship Restitution, had just arrived
at Salem, Massachusetts, where he observed the phenomenon from midnight
till daylight. He noticed with astonishment that precisely one year
before, viz., on the 13th of November, 1832, he had observed a similar
appearance (although the meteors were less numerous) at Mocha, in
Arabia. It was soon found, moreover, as a further and most remarkable
coincidence, that an extraordinary fall of meteors had been witnessed
on the 12th of November, 1799. This was seen and described by Andrew
Ellicott, Esq., who was then at sea near Cape Florida. It was also
observed in Cumana, South America, by Humboldt, who states that it
was "simultaneously seen in the new continent, from the equator to
New Herrnhut, in Greenland (lat. 64° 14´), and between 46° and 82°
longitude."
This wonderful correspondence of dates excited a very lively interest
throughout the scientific world. It was inferred that a recurrence
of the phenomenon might be expected, and accordingly arrangements
were made for systematic observations on the 12th, 13th, and 14th of
November. The periodicity of the shower was thus, in a very short
time, placed wholly beyond question. The examination of old historical
records led to the discovery of at least 12 appearances of the November
shower previous to the great fall of 1833. The descriptions of these
phenomena will be found collected in an interesting article by Prof.
H. A. Newton, in the _American Journal of Science and Arts_, for May,
1864. They occurred in the years 902, 931, 934, 1002, 1101, 1202,
1366, 1533, 1602, 1698, 1799, and 1832. Besides these 12 enumerated by
Professor Newton as "the predecessors of the great exhibition on the
morning of November 13th, 1833," we find 6 others, less distinctly
marked, in the catalogue of M. Quetelet.[2] These were in the years
1787, 1818, 1822, 1823, 1828, and 1831. From 1883 to 1849, inclusive,
Quetelet's catalogue indicates 11 partial returns of the November
shower; making in all, up to the latter date, 29. In 1835, November
13th, a straw roof was set on fire by a meteoric fire-ball, in the
department de l'Aine, France. On the 12th of November, 1837, "at 8
o'clock in the evening, the attention of observers in various parts
of Great Britain was directed to a bright luminous body, apparently
proceeding from the North, which, after making a rapid descent, in the
manner of a rocket, suddenly burst, and scattering its particles into
various beautiful forms, vanished in the atmosphere. This was succeeded
by others all similar to the first, both in shape and the manner of its
ultimate disappearance. The whole display terminated at ten o'clock,
when dark clouds, which continued up till a late hour, overspread the
earth, preventing any further observations."--_Milner's Gallery of
Nature_, p. 142.
In 1838, November 12th-13th, meteors were observed in unusual numbers
at Vienna. One of extraordinary brilliancy, having an apparent
magnitude equal to that of the full moon, was seen near Cherburg.
On several other returns of the November epoch the number of meteors
observed has been greater than on ordinary nights; the distinctly
marked exhibitions, however, up to 1866, have all been enumerated.
THE SHOWER OF NOVEMBER 14, 1866.
The fact that all great displays of the November meteors have
taken place at intervals of thirty-three or thirty-four years, or
some multiple of that period, had led to a general expectation of
a brilliant shower in 1866. In this country, however, the public
curiosity was much disappointed. The numbers seen were greater than
on ordinary nights, but not such as would have attracted any special
attention. The greatest number recorded at any one station was seen at
New Haven, by Prof. Newton. On the night of the 12th, 694 were counted
in five hours and twenty minutes, and on the following night, 881
in five hours. This was about six times the ordinary number. A more
brilliant display was, however, witnessed in Europe. Meteors began
to appear in unusual frequency about eleven o'clock on the night of
the 13th, and continued to increase with great rapidity for more than
two hours; the maximum being reached a little after one o'clock. The
Edinburgh _Scotsman_, of November 14th, contains a highly interesting
description of the phenomenon as observed at that city. "Standing on
the Calton Hill, and looking westward," the editor remarks,--"with
the Observatory shutting out the lights of Prince's Street--it was
easy for the eye to delude the imagination into fancying some distant
enemy bombarding Edinburgh Castle from long range; and the occasional
cessation of the shower for a few seconds, only to break out again with
more numerous and more brilliant drops of fire, served to countenance
this fancy. Again, turning eastward, it was possible now and then to
catch broken glimpses of the train of one of the meteors through the
grim dark pillars of that ruin of most successful manufacture, the
National Monument; and in fact from no point in or out of the city was
it possible to watch the strange rain of stars, pervading as it did all
points of the heavens, without pleased interest, and a kindling of the
imagination, and often a touch of deeper feeling that bordered on awe.
The spectacle, of which the loftiest and most elaborate description
could but be at the best imperfect--which truly should have been seen
to be imagined--will not soon pass from the memories of those to whose
minds were last night presented the mysterious activities and boundless
fecundities of that universe of the heavens, the very unchangeableness
of whose beauty has to many made it monotonous and of no interest."
The appearance of the phenomenon, as witnessed at London, is minutely
described in the _Times_ of November 15th. The shower occurred chiefly
between the hours of twelve and two. About one o'clock a single
observer counted 200 in two minutes. The whole number seen at Greenwich
was 8485. The shower was also observed in different countries on the
continent.
_The Meteors of 1866 compared with those of former Displays._
The star shower of 1866 was much inferior to those of 1799 and
1833.[3] With these exceptions, however, it has, perhaps, been
scarcely surpassed during the last 500 years. Historians represent the
meteors of 902 as innumerable, and as moving like rain in all possible
directions.[4] The exhibition of 1202 was no less magnificent. The
stars, it is said, were seen to dash against each other like swarms of
locusts; the phenomenon lasting till daybreak.[5] The shower of 1366
is thus described in a Portuguese chronicle, quoted by Humboldt: "In
the year 1366, twenty-two days of the month of October being past,
three months before the death of the king, Dom Pedro (of Portugal),
there was in the heavens a movement of stars, such as men never before
saw or heard of. At midnight, and for some time after, all the stars
moved from the east to the west; and after being collected together,
they began to move, some in one direction, and others in another.
And afterward they fell from the sky in such numbers, and so thickly
together, that as they descended low in the air, they seemed large and
fiery, and the sky and the air seemed to be in flames, and even the
earth appeared as if ready to take fire. That portion of the sky where
there were no stars, seemed to be divided into many parts, and this
lasted for a long time."
The following is Humboldt's description of the shower of 1799, as
witnessed by himself and Bonpland, in Cumana, South America: "From half
after two, the most extraordinary luminous meteors were seen toward the
east.... Thousands of bolides and falling stars succeeded each other
during four hours. They filled a space in the sky extending from the
true east 30° toward the north and south. In an amplitude of 60° the
meteors were seen to rise above the horizon at E. N. E. and at E.,
describe arcs more or less extended, and fall toward the south, after
having followed the direction of the meridian. Some of them attained a
height of 40°, and all exceeded 25° or 30°.... Mr. Bonpland relates,
that from the beginning of the phenomenon there was not a space in
the firmament equal in extent to three diameters of the moon, that
was not filled at every instant with bolides and falling-stars....
The Guaiqueries in the Indian suburb came out and asserted that the
firework had begun at one o'clock.... The phenomenon ceased by degrees
after four o'clock, and the bolides and falling-stars became less
frequent; but we still distinguished some toward the northeast a
quarter of an hour after sunrise."
DISCUSSION OF THE PHENOMENA.
Since the memorable display of November 13th, 1833, the phenomena of
shooting-stars have been observed and discussed by Brandes, Benzenberg,
Olbers, Saigey, Heis, Olmsted, Herrick, Twining, Newton, Greg, and many
others. In the elaborate paper of Professor Olmsted, it was shown that
the meteors had their origin at a distance of more than 2000 miles
from the earth's surface; that their paths diverged from a common point
near the star _Gamma Leonis_; that in a number of instances they became
visible about 80 miles from the earth's surface; that their velocity
was comparable to that of the earth in its orbit; and that in some
cases their extinction occurred at an elevation of 30 miles. It was
inferred, moreover, that they consisted of combustible matter which
took fire and was consumed in passing through the atmosphere; that this
matter was derived from a nebulous body revolving round the sun in an
elliptical orbit, but little inclined to the plane of the ecliptic;
that its aphelion was near that point of the earth's orbit through
which we annually pass about the 13th of November--the perihelion being
a little within the orbit of Mercury; and finally that its period was
about one-half that of the earth. Dr. Olmsted subsequently modified his
theory, having been led by further observations to regard the zodiacal
light as the nebulous body from which the shooting-stars are derived.
The latter hypothesis was also adopted by the celebrated Biot.
The fact that the position of the radiant point does not change with
the earth's rotation, places the cosmical origin of the meteors wholly
beyond question. The theory of a closed ring of nebulous matter
revolving round the sun in an elliptical orbit which intersects that
of the earth, affords a simple and satisfactory explanation of the
phenomena. This theory was adopted by Humboldt, Arago, and others,
shortly after the occurrence of the meteoric shower of 1833. That the
body which furnishes the material of these meteors moves in a closed
or elliptical orbit is evident from the periodicity of the shower. It
is also manifest from the partial recurrence of the phenomenon from
year to year, that the matter is diffused around the orbit; while the
extraordinary falls of 1833, 1799, 1366, and 1202, prove the diffusion
to be far from uniform.
ELEMENTS OF THE ORBIT.
Future observations, it may be hoped, will ultimately lead to an
accurate determination of the elements of this ring: many years,
however, will probably elapse before all the circumstances of its
motion can be satisfactorily known. Professor Newton, of Yale College,
has led the way in an able discussion of the observations.[6] He has
shown that the different parts of the ring are, in all probability, of
very unequal density; that the motion is retrograde; and that the time,
during which the meteors complete a revolution about the sun, must be
limited to one of five accurately determined periods, viz.: 180·05
days, 185·54 days, 354·62 days, 376·5 days, or 33·25 years. He makes
the inclination of the ring to the ecliptic about 17°. The five periods
specified, he remarks, "are not all equally probable. Some of the
members of the group which visited us last November [1863] gave us the
means of locating approximately the central point of the region from
which the paths diverge. Mr. G. A. Nolen has, by graphical processes
specially devised for the purpose, found its longitude to be 142°, and
its latitude 8° 30´. This longitude is very nearly that of the point
in the ecliptic toward which the earth is moving. Hence the point
from which the absolute motion of the bodies is directed (being in a
great circle through the other two points) has the same longitude. The
absolute motion of each meteor, then, is directed very nearly at right
angles to a line from it to the sun, the deviation being probably not
more than two or three degrees.
"Now, if in one year the group make 2 ± 1/33·25 revolutions, there is
only a small portion of the orbit near the aphelion which fulfills the
above condition. In like manner, if the periodic time is 33·25 years,
only a small portion of the orbit near the perihelion fulfills it.
On the other hand, if the annual motion is 1 ± 1/33·25 revolutions,
the required condition is answered through a large part of the orbit.
Inasmuch as no reason appears why the earth should meet a group near
its apsides rather than elsewhere, we must regard it as more probable
that the group makes in one year either 1 + 1/33·25, or 1 - 1/33·25
revolutions."
Professor Newton concludes that the third of the above-mentioned
periods, viz., 354·62 days, combines the greatest amount of probability
of being the true one. We grant the force of the reasons assigned for
its adoption. At least one consideration, however, in favor of the
long period of 33·25 years is by no means destitute of weight: of
nearly 100 known bodies which revolve about the sun in orbits of small
eccentricity, not one has a retrograde motion. Now if this striking
fact has resulted from a general cause, how shall we account for the
backward motion of a meteoric ring, in an orbit almost circular, and
but little inclined to the plane of the ecliptic? In such a case, is
not the preponderance of probability in favor of the longer period?
A revolution in 33·25 years corresponds to an ellipse whose major axis
is 20·6. Consequently the aphelion distance would be somewhat greater
than the mean distance of Uranus. It may also be worthy of note, that
five periods of the ring would be very nearly equal to two of Uranus.
The _Monthly Notices of the Royal Astronomical Society_ for December,
1866, and January, 1867, contain numerous articles on the star shower
of November 13th-14th, 1866. Sir John Herschel carefully observed the
phenomena, and his conclusions in regard to the orbit are confirmatory
of those of Professor Newton. "We are constrained to conclude," he
remarks, "that the true line of direction, in space of each meteor's
flight, lay in a plane at right angles to the earth's radius vector at
the moment; and that therefore, except in the improbable assumption
that the meteor was at that moment _in perihelio_ or _in aphelio_, its
orbit would not deviate greatly from the circular form." The question
is one to be decided by observation, and the only meteor whose track
and time of flight seem to have been well observed, is that described
by Professor Newton in _Silliman's Journal_ for January, 1867, p. 86.
The velocity in this case, if the estimated time of flight was nearly
correct, was _inconsistent with the theory of a circular orbit_.
It is also worthy of notice that Dr. Oppolzer's elements of the first
comet of 1866 resemble, in a remarkable manner, those of the meteoric
ring, supposing the latter to have a period of about 33-1/4 years.
Schiaparelli's elements of the November ring, and Oppolzer's elements
of the comet of 1866, are as follows:
November Comet of
Meteors. 1866.
Longitude of perihelion 56° 25´ 60° 28´
Longitude of ascending node. 231 28 231 26
Inclination 17 44 17 18
Perihelion distance 0·9873 0·9765
Eccentricity 0·9046 0·9054
Semi-axis major 10·3400 10·3240
Period, in years 33·2500 33·1760
Motion Retrograde. Retrograde.
It seems very improbable that these coincidences should be accidental.
Leverrier and other astronomers have found elements of the meteoric
orbit agreeing closely with those given by Schiaparelli. Should the
identity of the orbits be fully confirmed, it will follow that the
comet of 1866 _is a very large meteor_ of the November stream.
The researches of Professor C. Bruhns, of Leipzig, in regard to this
group of meteors afford a probable explanation of the division of
Biela's comet--a phenomenon which has greatly perplexed astronomers for
the last twenty years. Adopting the period of 33-1/4 years, Professor
Bruhns finds that the comet passed extremely near, and probably
_through_ the meteoric ring near the last of December, 1845. It is easy
to perceive that such a collision might produce the separation soon
afterward observed.
As the comet of Biela makes three revolutions in twenty years, it was
again at this intersection, or approximate intersection of orbits about
the end of 1865. But although the comet's position, with respect to
the earth, was the same as in 1845-6, and although astronomers watched
eagerly for its appearance, their search was unsuccessful. In short,
_the comet is lost_. The denser portion of the meteoric stream was then
approaching its perihelion. A portion of the arc had even passed that
point, as a meteoric shower was observed at Greenwich on the 13th of
November, 1865.[7] The motion of the meteoric stream is retrograde;
that of the comet, direct. Did the latter plunge into the former, and
was its non appearance the result of such collision and entanglement?
[Illustration: Fig. 3.
_Probable Orbit of the November Meteors._]